Approximately 80% of the world's power is generated by fossil fuel combustion. In the United States, more than 90% of greenhouse gas emissions come from the combustion of fossil fuels. Combustion of fossil fuels also produces other air pollutants, such as nitrogen oxides, sulfur dioxide, volatile organic compounds and heavy metals. Additionally, the cost of producing energy is skyrocketing, due in part to depleting fossil fuel reserves. Clean, renewable energy sources are required to meet upcoming energy demands.
Light energy is characterized by a dual nature both from a quantum point of view as photons and from a wave point of view as randomly polarized electromagnetic radiation with a wavelength between 400 nm and 700 nm. If the ultraviolet and infrared portion of the spectrum is included, the range of wavelengths is extended at both extremes. Presently, all practical solar cell energy collection schemes utilize the photon nature of light. For example, the conversion of solar energy to electrical energy using the photovoltaic effect depends upon the interaction of photons with energy equal to or greater than the band-gap of the rectifying material. With continued research, the maximum amount of energy captured using the photovoltaic mechanism is estimated to be around 30%.
A rectenna is a combination of an antenna and a rectifier (diode). Rectennas have been studied mainly for microwave-based power transmission, with efficiencies exceeding 80% at 2.5 GHz [W. C. Brown, “The history of power transmission by radio waves” IEEE Trans. Microwave Theory and Techn., 32:9, 1230-1242, September 1984), with the devices used to directly convert microwave energy into DC electricity. Its elements are usually arranged in a multi element phased array with a mesh pattern reflector element. Rectennas are highly efficient at converting microwave energy to electricity, with observed efficiencies above 90%. Optical rectennas harvest solar energy and convert it into electric power. Optical rectennas consist of an optical antenna to efficiently absorb the incident solar radiation and a high-frequency metal-insulator-metal (MIM) tunneling diode that rectifies the AC field across the antenna, providing DC power to an external load. The combination of a rectifying diode at the feedpoints of a receiving antenna is often referred to as a rectenna. Utilizing a rectenna to harvest solar energy relies upon the electromagnetic nature of radiation and is not limited by the band-gap of the rectifying material. As such, this method is not fundamentally band-gap limited. At microwave frequencies (˜2.4 GHz) the rectenna approach has been demonstrated to be approximately 90% efficient. Rather than generating electron-hole pairs as in the photovoltaic method, the electric field from an incident electromagnetic radiation source will induce a wave of accelerated electric charge in a conductor. Efficient collection of the incident radiation is then dependent upon resonance length scales and impedance matching of the collecting antenna to the rectifying diode to minimize losses. However, methods of harvesting high-frequency radiation utilizing rectennas have identified several key problems with the approach. These problems include impedance matching, rectification, polarization, limited bandwidth and captured power.
Traditionally, the λ/2 dipole antenna is the most commonly used antenna by the designer as the receiving device for a rectenna due to the straightforward design procedure and the ease of fabrication as a printed circuit antenna. However, the λ/2 dipole has shortcomings, such as only supporting a single polarization. It exhibits a relatively low gain, it exhibits very high conductor losses at higher frequencies and its radiation pattern is omni-directional. It has been shown that the rectifier efficiency would be less than 0.1% for the calculated power at the terminal of a rectenna utilizing a λ/2 dipole antenna.
Polarization of solar radiation is known to be random (unpolarized). An unpolarized electromagnetic wave is a collection of waves that have an equal distribution of electric field orientations in all directions. A randomly polarized wave can be decomposed into two main components, Ex, and Ey. The typical λ/2 dipole antenna only supports a single polarization and is therefore not useful for the collection of solar radiation or other unpolarized electromagnetic energy.
Accordingly, what is needed in the art is an improved rectenna for the collection of electromagnetic energy and more particularly an improved rectenna for the collection of solar energy that overcomes the identified deficiencies in the prior art solutions.
The antenna coupled Metal-Insulator-Metal (MIM) detector is a device that can operate at ambient temperature and can be used as detector and harmonic mixer up to 150 THz (D. A. Jennings, et al., Appl. Phys. Lett. 26 (1975) 510). In a MIM tunnel diode, the electrons flow between the metal electrodes via an ultra-thin insulator layer (S. M. Sze, Physics of Semiconductor Devices, 2nd Ed., Wiley Inter-Science Publication, New York, 1981, pp. 553-558).
Current work has focused on using similar devices, scaled down in size using nanotechnology, to convert light into electricity at much greater efficiencies than what is currently possible with traditional solar cells. Because of limitations in nanotechnology fabrication, it has not been possible to develop rectennas that can operate in the visible frequency range. Theoretically, high efficiencies can be maintained as the device shrinks, but current optical rectennas have only obtained roughly 1% efficiency using light. Current work with infrared rectennas has created more efficient energy collection devices.